1. Field of the Invention
The present invention relates generally to a multilayer capacitor having a structure in which a desired capacitance can be achieved, and more particularly, to a multilayer capacitor which is provided with a correction electrode for correcting the capacitance of the multilayer capacitor.
2. Description of the Prior Art
It is well-known that in a multilayer capacitor, a plurality of inner electrodes can be arranged so as to overlap each other while being separated by dielectric ceramic layers. In such a multilayer capacitor, usually the number of the dielectric ceramic layers sandwiched between the inner electrodes is adjusted in order to obtain a desired capacitance. This number of dielectric ceramic layers n is generally calculated by the following equation: EQU n=(113.times.C.multidot.t)/(.epsilon..multidot.S)
where C denotes a capacitance value to be achieved (pF), t denotes the thickness of dielectric ceramic layer (mm), .epsilon. denotes a dielectric constant, and S denotes the effective overlapping area of the inner electrodes (mm.sup.2).
As the result of making calculations using the foregoing equation so as to achieve a certain capacitance value C, however, the problem may rise that n is not an integer, for example, n=8.3.
When n is not an integer, the desired capacitance value C has been conventionally achieved by adopting the following methods.
First, a method has been adopted wherein n is rounded and a lot of multilayer capacitors having an integral number of layers formed therein is fabricated. Then a multilayer capacitor having the desired capacitance value C is selected from among the lot of multilayer capacitors fabricated. However, the center value of the distribution of capacitance values of the multilayer capacitors obtained by this method can be expected to deviate considerably from the desired capacitance value C. Consequently, the method has the disadvantage that the rate of acceptable products is significantly low.
Second, as shown in FIG. 2, the thickness may be increased of a dielectric ceramic layer between an inner electrode 5 and an inner electrode 6, for example, a plurality of inner electrodes 2 to 6 being arranged in a sintered body 1. For example, when n=8.3, a capacitance value close to the capacitance value C of the multilayer capacitor in which n=8.3 can be obtained by laminating the inner electrodes 2 to 5 whose number corresponds to the number of layers n=8.0 (some of the inner electrodes are omitted in FIG. 2) and interposing between the inner electrodes 5 and 6 a dielectric ceramic layer 7 having a thickness which is three times that of the dielectric ceramic layers between the other inner electrodes.
However, even if the above described dielectric ceramic layer 7 and inner electrode 6 are used, the desired capacitance value C corresponding to n=8.3 cannot be directly achieved. Accordingly, the method has the disadvantage that the rate of acceptable products is not satisfactory. Moreover, it must be arranged to provide the dielectric ceramic layer 7 having a different thickness from that of the other dielectric ceramic layers. Consequently, in the fabrication, a ceramic green sheet different from the other ceramic green sheets (having no electrode pattern formed therein) must be inserted. Therefore, the method also has the disadvantage that the fabricating processes are complicated.
Third, a structure has been known which includes an auxiliary electrode 16, as shown in FIGS. 3A and 3B. In this structure, inner electrodes 12 to 15 required to achieve a capacitance value in the case of n=8 are arranged in a ceramic sintered body 11. The auxiliary electrode 16 is provided which overlaps the inner electrode 15 while being separated therefrom by a dielectric ceramic layer. The auxiliary electrode 16 is constructed with a width equal to that of the other inner electrodes 12 to 15, but a length smaller than that of the other inner electrodes, as shown in FIG. 3B which is a longitudinal cross-section viewed from below, so that the overlapping area between the auxiliary electrode 16 and the inner electrode 15 becomes three-tenths of the overlapping area between the other inner electrodes.
Consequently, in the structure using the auxiliary electrode 16, a multilayer capacitor having a capacitance corresponding to the capacitance value C in the case where n=8.3 is obtained.
However, the structure using the auxiliary electrode 16 shown FIG. 3B has the disadvantage in that a multilayer capacitor having a desired capacitance cannot be obtained with high precision and stability because the capacitance value fluctuates very greatly if the position where the auxiliary electrode 16 is formed is slightly shifted in the direction of width W.
Furthermore, the dimensions of the multilayer capacitor are, in general, extremely small, and specifically the distance between the end surfaces of the sintered body 11 of the multilayer capacitor 10 is short. In such a very small component, therefore, it is extremely difficult to precisely overlap the auxiliary electrode 16 with the inner electrode 15 by a distance equal to 0.3.times. as shown in FIG. 3B, in that the overlapping area varies substantially in the direction of length L.
Therefore, even if a multilayer capacitor is fabricated so as to achieve as its objective the capacitance value corresponding to the case of n=8.3, the capacitance value of the multilayer capacitor obtained is nevertheless will vary over an appreciable range.